Flow of a liquid containing gelatin particles to similate red blood cells will be investigated experimentally in collapsible models of the alveolar capillary network. Specifically the following questions will be investigated: (1) What is the effect of external pressure on the mode of tube collapse in the network? (2) Do preferred channels develop? (3) What are the relative roles of recruitment and distensibility as transmural pressure is varied? (4) What is the effect on the flow of changes in Hematocrit? (5) What is the effect on the flow of changes in network geometry? The ultimate research objectives are (1) to provide a sound physical basis for the understanding of the effects of positive end expiratory pressure ventilation (PEEP) on blood flow in the alveolar capillaries, and (2) to bring together in a more realistic model the best qualities of the Starling resistor collapsible tube concept and the rigid sheet flow model.